Amplifiers are devices that accept a varying input signal and produce an output signal that varies in the same way as the input, but with larger amplitude. The input and output signals may consist of a current, a voltage, a mechanical motion, or any other signal. An electronic amplifier is a device for increasing the power of a signal. It does this by taking power from a power supply and shaping the output to match the input signal. This process invariably introduces some noise and distortion into the signal, and the process is not completely efficient. Amplifiers always produce some waste as heat.
Different designs of amplifiers are used for different types of applications and signals. Amplifiers broadly fall into three categories: small signal amplifiers, low frequency power amplifiers, and RF power amplifiers. The most common types of amplifiers are electronic and have transistors or electron tubes as their principal components. Electronic amplifiers are widely used in consumer electronic devices, such as in radio and television transmitters and receivers, as well as audio and stereo systems.
In general, commercial audio power amplifiers are valued according to their power rating into various load impedances. Whether the sound system is in a stadium, nightclub, or a maximum sound pressure level (“SPL”) automotive competition, value is measured in SPL, and more SPL requires more power. Conventional amplifier technology will produce the highest power rating into a single rated load impedance only, which is the lowest impedance. At higher load impedances, the power rating decreases. When selecting a power amplifier for a set of speakers, a system designer will typically choose an amplifier that has sufficient power at the speakers' nominal impedance, which is also normally the speakers' lowest impedance. Considering the wide variation of impedance with frequency commonly seen in individual speakers and loudspeaker systems, it is not unusual to find that the conventional amplifier can only deliver maximum rated power over a narrow range of frequencies surrounding the lowest point in the speaker impedance curve, which is the nominal impedance.
There have been a few examples of commercially available amplifiers that use manual or automatic means to deliver rated power over a range of rated nominal speaker impedances. One example of these quasi-constant power amplifiers is a single-ended triode vacuum-tube power amplifier that incorporated a step-down transformer in the output stage to impedance match the high voltage output of the tubes to the relatively low impedance of speakers. In order to deliver rated power into a variety of nominal impedances, typically between 8Ω to 16Ω, the output transformer had a choice of “taps” where different step-down winding ratios could be selected to match the output voltage of the vacuum tubes to the exact voltage that would produce rated power into the desired speaker impedance.
Other examples of manually corrected quasi-constant power amplifiers employed a panel-mounted “impedance switch” where the user could indicate a 2Ω, 4Ω, or 8Ω nominal impedance speaker was being used. The amplifier would then adjust its internal switch-mode-power-supply to the correct voltage, such that the output stage will be limited to a voltage that will produce no more than rated power at the user-specified nominal impedance. In other variations of this type of quasi-constant power amplifier, the “impedance switch” is automated by internal circuits that measure the nominal or minimum speaker impedance.
These quasi-constant power amplifiers represented improvements in the amplifier-speaker matching process by giving the user a versatile amplifier that could be set up manually or automatically to a single discrete choice that would deliver maximum rated power at the lowest expected or nominal load impedance. However, none of these amplifiers can react to a dynamically changing load impedance, such as a complex loudspeaker load driven with music and automatically limit at rated power over the range of rated impedances. These quasi-constant power amplifiers were optimized for one impedance only, which was the nominal impedance, so the conventional amplifier problem of only achieving rated power over the narrow range of frequencies surrounding the nominal impedance was not solved.
Accordingly, what is needed is a new power amplifier that delivers maximum rated power into all rated impedances.